Method of making low sulfur steel



July 30, 1963 T. w. LANDlG ETAL 3,

METHOD OF MAKING LOW SULFUR STEEL Filed Oct. 21, 1958 Mal/en 3/0 Molten Iron 5 0.2 Mn 0. 5 c 0.3 De oxidizing Slag l'ron Graghife Lime Silicon I Aluminum Desu/fur/zmg Slag Iron Oxidizing Oxygen Slag Iron Product 5 =0.005 Mn=O- 02 C 0 02 5/ Res/dual /nvenf0rs: Thomas W Land/g; Al/an J. K/es/er,

3,099,552 Patented July 30, 1963 3,099,552 METHOD OF MAKING LOW SULFUR STEEL Thomas W. Landig, Glenshaw, Pa., and Allan J. Kiesler,

Schenectady, N .Y., assignors to General Electric Company, a corporation of New York Filed Oct. 21, 1958, Ser. No. 768,601 2 Claims. (CI. 75-51) The present invention relates generally to the art of refining metals and is more particularly concerned with a novel method for desuifurizing ferrous metals having special important advantages and utility.

It has long been recognized by those skilled in the art that there is a real need for some method or means by which the sulfur content of iron and steel could be rapidly and economically reduced to the region of 0.005%. At the present time, however, for a lack of a better method it is standard practice in the steel industry to carry out desulfurization in a way which precludes the production at reasonable costs of metal answering the sulfur requirements of specialty alloys such as silicon irons and stainless steels.

It is accordingly a primary object of the present invention to provide a commercially feasible method enabling the reduction of sulfur in ferrous metals to 0.005% or less.

It is a further important object of this invention to provide a method for thus desulfurizing ferrous metals which will not necessitate costly or impracticable departures from presently standard practices.

It is still another important object of this invention to provide an answer to the foregoing problem without time incurring any substantial oif-setting disadvantage.

Other objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description, reference being had to the drawing accompanying and forming a part of this specification.

As illustrated in the flow-sheet drawing, the process or method of this invention comprises very generally the steps of deoxidizing, then desulfurizing, and finally oxidizing a ferrous metal to be refined or purified in respect to carbon, silicon, manganese and sulfur. It is essential that these operations or steps be carried out separately and in the sequence described. Thus, for example, a change in the sequence of the second and final steps would make this process correspond to presently conventional practice and would prevent the realization of the new advantages and the obtaining of the new results of this invention, as set forth above.

In its broadest aspect the present invention involves the desulfurizing of molten iron or steel, followed by the oxidation and removal of certain elements which are now removed prior to the desulfurizing step. By virtue of this departure in principle from the prior art, substantial reducing effects of elements such as carbon, manganese, silicon and aluminum in the metal are obtained to promote the speedy and virtually complete removal of sulfur.

In the preferred practice of the process of this invention ferrous metal containing undesired amounts of impurities including carbon, silicon, manganese and sulfur is melted down then deoxidized, and then desulfurized to bring its sulfur content to the desired level, and finally the metal melt is oxidized until the manganese and carbon contents have been reduced to a predetermined desired level. Ordinarily phosphorus will also be a substantial impurity to be removed and in the course of the deoxidizing of the freshly prepared melt, the phosphorous content of the metal will be reduced to a predetermined minimum. Actually, reduction of the metal content of both phosphorus and FeO will go on substantially simultaneously so that at the end of the deoxidizing operation the metal will be purified of both these contaminants to the extent desired. The metal at this stage will contain less than 0.010% phosphorus. In the next purification step the sulfur content of the melt will be reduced to about 0.005%. And in the final purification step the manganese content and the carbon content of the metal will each have been reduced to approximately 0.02%.

As the first step in the usual operation of the process of this invention, the metal to be purified is melted down with conventional slagging components designed to enable effective removal of phosphorus and FeO. This melting down operation may be carried out in any suitable vessel and preferably one of design which will readily permit the clean separation of slag from the metal. As soon as the FeO and phosphorus contents of the metal melt have been reduced to their predetermined levels, the slag is separated from the metal, care being exercised to remove the metal as cleanly as possible from the slag so as to prevent transfer back of FeO- or phosphorus or both in later steps of the process.

Following the separation of the first slag from the metal, a second slag is prepared in contact with the metal by bringing together lime, finely divided silicon and graphite and mixing these materials in contact with the metal. The addition of 0.2% to 0.3% aluminum pig to the heat may be made at this point to aid in the desulfurizing operation. This second slag is then worked into the heat by appropriate mixing operations or, for example, by reladling so as to effect rapid and complete sulfur removal. This carbide slag is then carefully substantially completely removed and separated from the metal melt in order to prevent re-entry of sulfur or reversion of sulfur to the metal during subsequent processing.

As the final step of the process of purifying the iron or steel melt, oxygen or air or other oxygen-containing gas is delivered into the iron so as to oxidize the carbon, manganese, sulfur, silicon and aluminum therein. During this blowing operation, the metal is protected from contact from sulfur-bearing materials and the sulfur content of the metal is thus maintained at the minimum established in the desulfurizing operation. Depending upon the amounts of the various oxidiza-ble impurities noted above, the blowing operation will be continued for a length of time sufiicient to accomplish the removal of these elements to the desired extent.

Followin this oxidation step, the metal is treated as required and tapped and cast suitably in accordance with generally conventional practice. A tapping temperature in the range of 2800 F. to 3100" F. may be suitable for certain types of metal as those skilled in the art will understand and variations from this temperature range are within reason contemplated in the ordinary use of this process. A small amount of lime may be added for the purpose of holding back the slag at the tapping time and alloying additions may be made to the metal in the ladle, if desired.

Those skilled in the art may gain a further and better understanding of the essential features and the preferred practice of the present invention on consideration of the following illustrative, but not limiting, example.

Example 1 Iron containing 0.50% silicon, 0.50% manganese, 0.30% of carbon and from 0.015% to 0.020% sulfur is charged into a melting furnace and melted down with slagging ingredients to produce a metal product containing not more than 0.010% of phosphorus. Following clean separation of the first slag containing FeO and phosphorus from the metal melt, the metal is treated for the removal of sulfur, two parts of lime, one part of fine silicon and one part of Mexican graphite being added to the metal and mixed with it. Fluo-rspar is added to the melt at the same time in an amount suitable to provide the desired viscosity in the slag material. This second slag is the carbide slag and it is effective together with the carbon, manganese, silicon and aluminum present at this stage and an amount of 0.2% to 0.3% aluminum added in pig form to eifect desulfurizing and removal of sulfur from the metal to the slag. Mixing of the slag with the metal is carried out so as to promote the cleansing of the metal of sulfur and this is suitably done by a reladling operation where the volumes of metal are relatively small. As soon as the sulfur content of metal has reached the desired level, preferably less than about 0.005%, the carbide slag is removed from the metal and the bath or metal surface is skinned as cleanly as possible to get rid of all of the sulfur-containing slag. In this Way the tendency for sulfur reversion to occur with resultant contamination of the metal with sulfur previously removed is avoided. From this point on in the process, special care is taken to avoid contact of the metal with any sulfurbearing material.

The metal is next treated for the removal of aluminum, silicon, manganese and carbon, being blown with oxygen for this purpose. This step of the process is carried out rapidly with the analysis of the metal being as stated above and the contents of the impurities to be removed accordingly being relatively small so that the oxidation reactions necessarily go on to the extent desired in a very short period. In this case the chemistry of the metal at the time that it is tapped is 0.03% carbon, 0.02% manganese, 0.005% sulfur and less than 0.01% aluminum. The silicontent is residual. This metal is tapped at a temperature of 2840 F. to 2860" F. and a small addition of lime is made as indicated above for purposes of holding back slag during the tapping operation.

The melt down slag will suitably be made from limestone :and the oxides of silicon, iron and other elements having an affinity for lime under the circumstances. Accordingly, it will normally be satisfactory in carrying out the melt down operation to charge limestone in adequate amount together with the metal to be treated and to heat the limestone and the metal to produce a molten mass of metal with a molten slag cover.

Having thus described this invention in such full, clear, concise and exact terms as to enable any person skilled in the art to which it pertains to make and use the same, and having set forth the best mode contemplated of carrying outthis invention, we state that the subject matter which we regard as being our invention is particularly pointed out and distinctly claimed in what is claimed, it being understood that equivalents or modifications of, or substitutions for, parts of the specifically described embodiments of the 4: invention may be made without departing from the scope of the invention [as set forth in what is claimed.

What we claim "as new and desire to secure by Letter Patent of the United States is:

1. The method of producing steel containing less than 0.005% sulfur which comprises the steps of melting down with lime a steel containing more 0.015% sulfur and amounts of phosphorous, silicon, and manganese greater than desired in the final steel product, separating and removing firom the resulting metal melt the resulting slag when the phosphorous content of the metal melt has been reduced to a predetermined level, forming a second slag on the metal melt by adding to the metal melt two parts of lime, one part of Mexican graphite and one part of fine silicon, mixing the second slag with the metal melt, separating and removing the second slag from the metal melt when the sulfur content of the said melt has been redueced to less than 0.005%, and then oxidizing manganese and silicon in the metal melt by blowing the melt with a free oxygen-containing gas, and discontinuing the blowing of the melt when the amounts of manganese and silicon in the metal have been reduced to the desired levels.

2. The method of producing steel containing less than 0.005% sulfur which comprises the steps of melting down with lime a steel containing phosphorous, silicon and manganese and more than 0.015% sulfur, separating and removing from the resulting metal melt the resulting slag when the phosphorous content of the metal melt has been reduced to a predetermined level, forming a carbide slag on the metal melt by adding lime and graphite and silicon to the said melt, mixing the carbide slag with the metal melt, and separating and removing the carbide slag from the metal melt when the sulfur content of the said melt has been reduced to less than 0.005

References Cited in the file of this patent UNITED STATES PATENTS 1,053,454 Thallner Feb. 18, 1913 2,110,067 Heuer Mar. 1, 1938 2,258,850 Eulenstein et a1 Oct. 14, 1941 2,662,819 Hofges et a1. Dec. 15, 1953 2,668,759 Tenenbaum Feb. 9, 1954 2,670,283 Soisson Feb. 23, 1954 2,693,411 Cremer Nov. 2, 1954 2,811,436 Heuer Oct. 29, 1957 FOREIGN PATENTS 395,158 Great Britain July 13, 1933 

1. THE METHOD OF PRODUCING STEEL CONTAINING LESS THAN 0.005% SULFUR WHICH COMPRISES THE STEPS OF MELTING DOWN WITH LINE A STEEL CONTAINING MORE THAN 0.015% SULFUR AND AMOUNTS OF PHOSPHOROUS, SILICON, AND MANGANESE GREATER THAN DESIRED IN THE FINAL STEEL PRODUCT, SEPARATING AND REMOVING FROM THE RESULTING METAL MELT THE RESULTING SLAG WHEN THE PHOSPHORUS CONTENT OF THE METAL MELT HAS BEEN REDUCED TO A PREDETERMINED LEVEL, FORMING A SECOND SLAG ON THE METAL MELT BY ADDING TO THE METAL MELT TWO PARTS OF LIME, ONE PART OF MEXICAN GRAPHITE AND ONE PART OF FINE SILICON, MIXING THE SECOND SLAG WITH THE METAL MELT, SPEARATING AND REMOVING THE SECOND SLAG FROM THE METAL MELT WHEN THE SULFUR CONTENT OF THE SAID MELT HAS BEEN REDUCED TO LESS THAN 0.005%, AND THEN OXIDIZING MANAGANESE AND SILICON IN THE METAL MELT BY BLOWING THE MELT WITH A FREE OXYGEN-CONTAINING GAS, AND DISCONTINUING THE BLOWING OF THE MOLT WHEN THE AMOUNTS OF MANGENESE AND SILICON IN THE METAL BEEN REDUCED TO THE DESIRED LEVELS. 